1. Field of the Invention
The invention relates to brightness control, and in particular to a brightness control circuit utilizing pulse width modulation technology.
2. Description of the Related Art
Currently many electronic devices use panels to display information, such as cellphone, digital camera, personal digital assistant (PDA) and global positioning system (GPS). In brighter environments, panels require brighter display that users can easily see the content of the panels. While in darker environments, less brightness is desirable to conserve power. In addition, users will not feel uncomfortable if the panel is adjusted according to the environmental brightness. Since most of the devices mentioned use batteries as main power, in normal conditions, voltage of the batteries gradually decreases with use. Thus, the electronic devices' panels need to operate in a specific voltage range to maintain and provide specific illumination.
FIG. 1 is a schematic diagram of conventional panel brightness control circuit 100. DC/DC converter 10 receives battery power 50 for conversion to a specific voltage, such as 9 volts, provided to constant current controller 20. Constant current controller 20 uses sensor 30 to control current output to photodiode 40, further controlling the brightness of photodiode 40. In a brighter environment, constant current controller 20 provides a stronger current to photodiode 40, providing brighter emission. In darker environments, constant current controller 20 provides a weaker current to photodiode 40, providing darker emission.
Such conventional methods require extra DC/DC converter 10 to provide constant voltage and extra constant current controller 20 to provide the constant current. Thus, the system is more complicated and costly.
In addition, conventional cameras have a light sensor disposed inside the lens, although brightness on the lens side and the panel side (back of lens) are different. The light sensor does not detect actual brightness of the panel side, often causing a problem wherein the panel of the conventional camera is too bright or too dark for a user and wastes excess electrical power.